Biogenic methane and burrowing as important controlling factors in the early diagenesis of permian carbonate rocks in South Sichuan/China
Tóm tắt
The 400m thick Lower Permian carbonate sequence of South Sichuan/China, which is considered to be a most important gas reservoir rock, exhibits calcitized gas bubbles and dolomitized burrows which are the result of early diagenesis. The fossiliferous, highly algal-enriched limestones are part of the Yangtze Platform and are composed of biomicrites, biosparites and calcarenites of subtidal and intertidal environments which underwent a maximum burial depth of some 6000 m. The calcitized or, rarely, chert-filled gas bubbles represent a morphogenetic series ranging from a slightly brecciated sediment to globular forms and vertically elongated to branched dm-sized vugs; the series contains some internal sediments (lumps, calcarenites) which are identical to the host rock. Vadose influence is missing. Marcasite aggregates precipitated at the beginning of calcite cement formation emphasize the reducing conditions whereas negative δ13C values (−0.6 to −2.5 ‰) indicate the participation of CO2 and H2O originating from the decay of organic matter. Calcite cements contain reduced amounts of Na. K, Sr, Ba and Mg but are enriched in Fe and Mn in comparison to the host rock. It is obvious from the data that rising biogenic gases (CH4, CO2, NH3, H2S) under different partial pressures formed the vugs within an unconsolidated sediment (water content approximately 80%). Burrows parallel to the bedding (typeThalassinoides), which contain biocalcarenites (echinoderm bioclasts) as internal sediment, underwent early diagenetic dolomitization. These dolomites, which rarely developing into dm-thick supratidal beds, document a slight evaporation within the intertidal environments. Surprisingly enough they reveal a slight over abundance of Mg (Ca47Mg53...) which can be explained by an influence of rising formation waters rich in Mg. In comparison to the host rock the dolomites are enriched in Fe, Mn, K, Na, Zn, Pb and As, and exhibit decreased amounts of Ba and Sr as well as slightly higher δ13C values. The vugs play an insignificant role in respect to reservoir properties, because of their isolated distribution and early cementation which destroyed the pore spaces. By contrast, the recrystallized early diagenetic dolomitized burrows exhibit intercrystalline pores which are often occupied by migrated bitumen, but they are not economically relevant for the reservoir properties of the whole sequence.
Tài liệu tham khảo
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